Toxicity studies with Firemaster FF-1 (mixture of polybrominated biphenyls) and pure 2,4,5,2',4',5'-hexabromobiphenyl (HBB) in rats and mice were conducted in two steps: (a) rats and mice of both sexes were given either compound via gavage (daily 5x/wk for 1 month) and were examined 15, 30, 45, 60 and 120 days post treatment; (b) Firemaster FF-1 only, was then given to rats and mice at various concentrations (total 125 daily doses, 5x/wk for 6 months). The following parameters were studied: (a) food consumption; (b) body weight gain; (c) hematology; (d) clinical pathology; (e) anatomic pathology; (f) thyroid function; (g) urinary and tissue porphyrins; and (h) liver drug metabolizing enzymes.